One of the advantages of optical fiber communication is the potential for large information handling capacity. One approach to increasing the optical bandwidth over which information is transmitted in an optical fiber is to use wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM), where light at several different wavelengths is combined and injected into a fiber, the light at each wavelength typically being independently modulated with information prior to combining with the other wavelengths. After propagation through the fiber, the light is then separated into its different wavelength components before detection.
As used herein, the term WDM includes DWDM. The International Telecommunications Union (ITU) has set different WDM standards, that specify the operating wavelengths for the different WDM components, also known as channels. Under these standards, the separation between adjacent WDM channels is typically a fixed frequency. For example the inter-channel spacing may be 100 GHz or 50 GHz.
More information may be transmitted over a fixed bandwidth when the channel separation is smaller, since more channels can fit into the fixed bandwidth. However, it becomes increasingly difficult to multiplex or demultiplex the WDM channels when the frequency separation is smaller. As the pressure for increased fiber information capacity increases, the requirements for that optical WDM components can handle increasing dense multiplexing also increases.
The transmission of a multiple channel signal along a fiber link often introduces wavelength dependent losses or gains, resulting in non-uniform channel power. It is important to be able to monitor the power in different channels using a channel monitor. It is also important to be able to adjust the relative power levels in the different optical channels.
Furthermore, a complex communications network typically does not consist only of point to point links, but includes one or more local loops branching off main trunk fibers. Such local loops permit smaller communities to be attached to the communications network while the trunk fiber passes between major cities. The trunk fiber includes add/drop multiplexers that select one or more channels propagating along the trunk fiber and that add channels to the trunk fiber. Several approaches to add/drop multiplexing are fixed, and permit no change in the selected channel.